1. Field of the Invention
This invention relates to the field of herbicidal compositions and particularly to systemic herbicidal compositions and methods for using such compositions to control vegetation.
2. Description of the Art
The unique herbicidal properties of glyphosate and several of its more soluble salts are renowned. As reported by Grossbard and Atkinson in "The Herbicide Glyphosate," Butterworths, Boston, 1985, and by J. E. Franz in U.S. Pat. No. 3,799,758, glyphosate is a broad spectrum, nonselective, post-emergent herbicide having high unit activity on a wide variety of both annual and perennial plants. It can be manufactured and applied as glyphosate per se or any one of a very broad variety of glyphosate derivatives and homologues which, according to Franz, include halogen; hydroxy; thio; ammonium; mono- and di-alkylamine; hydroxy-alkyl and alkenyl amine, hydrocarbyl, hydrocarbonoxyhydrocarbyl, halohydrocarbyl, and halo-hydrocarbonoxyhydrocarbyl esters and thioesters; aminohydrocarbyl; metallo-oxy including alkali and alkaline earth, copper, zinc, manganese and nickel-oxy; amminoxy; organic amminoxy; and/or strong acid salt derivatives and homologs. According to Franz, supra, the alkali, alkaline earth, ammonium and organic amine salts are preferred. The isopropylamine salt is marketed by Monsanto Chemical Company as Roundup.sup.R.
According to Grossbard et al., supra, when glyphosate is contacted with plant foliage, it is translocated to the plant roots, rhyzomes, and apical meristemes giving it its systemic property and resulting in the total destruction of many resistant perennial weeds such as rhyzome sorghum halepense, Agropyron repens, Cirsium arvense, Cyprus spp., C. dactylon, and others. Glyphosate, per se, has relatively broad herbicidal utility since it is active on a variety of vegetation. It is systemic, non-persistent, and is readily metabolized by soil microorganisms to form plant nutrients, including phosphoric acid, ammonia, and carbon dioxide. Thus, glyphosate is environmentally attractive in comparison to many alternative herbicides. For these reasons, glyphosate-containing herbicides are marketed in over 100 countries and are used to control undesired vegetation in crop lands, plantations, orchards, industrial and recreational areas, and for home use.
As with all things in nature, however, there is always room for improvement. While glyphosate is a very active, broad spectrum, systemic, relatively environmentally safe herbicide, its solubility in water at 25.degree. C. is only 1.2 weight percent and many of its homologues and salts are only slightly soluble or are essentially insoluble in water and organic solvents. For instance, Franz illustrated that the glyphosate-hydrochloric acid "salt" is essentially insoluble in either water or tetrahydrofuran. Glyphosate is expensive and, when applied at recommended dosages as the isopropylamine salt, it does not completely control all plant species and maximum control does not occur for 1 to 3 weeks depending on plant species, dosage, etc. Moreover, many of the organic amine salts preferred by Franz have negative environmental effects which are not exhibited by the parent compound itself. For instance, the isopropylamine salt is toxic to fish and is not approved, at least not in the United States, for use on aquatic vegetation. The active portion of the molecule in the deliverable (water soluble) compounds----the glyphosate segment----is relatively chemically unstable even in weakly basic environments. Thus, glyphosate is hydrolyzed in weak base which may account for its failure to control vegetation in some instances. Mineral dust which accumulates on vegetation is generally alkaline, and hydrolysis which can occur in that environment (presumably at the amide link in the compound) deactivates glyphosate as a herbicide.
Several investigators have found that the herbicidal activity of glyphosate and its compounds can be increased in certain respects by formulation with other compounds. For instance, Grossbard and Atkinson, supra, report at page 226, that, under suitable conditions, ammonium salts, such as ammonium sulfate, can increase the phytotoxicity of a variety of water-soluble leaf-applied herbicides, including certain water-soluble glyphosate derivatives. According to Grossbard et al., these effects are evident particularly when ammonium salts such as ammonium sulfate are combined with the appropriate surfactant (ibid., page 228). The effects of several other compounds have also been investigated. For instance, Grossbard et al., report, at page 229, that ammonium salts other than ammonium sulfate have shown improved herbicidal effects in isolated studies and that those effects are less than have been observed in other instances with ammonium sulfate. They also report that, in one test on C. rotundas, slight improvement in glyphosate activity was observed upon addition of urea to the formulation. Other investigators have studied the relative effects of hydrophilic and lipophilic surfactants in the presence or absence of other components such as ammonium salts (Grossbard et al., ibid., page 228). Polybasic acids such as orthophosphoric and oxalic acids reportedly have shown improvement for the control for Agropyron repens when used in combination with certain soluble glyphosate derivatives (Grossbard et al., supra, page 230). While the precise mode of action of the previously tested ammonium salts, surfactants, polybasic acids, and other additives is not known with certainty, it has been suggested that certain ammonium salts modify plant membrane permeability but do not appear to directly influence translocation, while the polybasic acids may improve activity of the isopropylamine salt by sequestering and/or immobilizing metals such as calcium (ibid, page 230). However, the herbicidal effectiveness, per se, of glyphosate compounds and compositions, according to Franz, is not affected by the type of salt, i.e., the glyphosate counter-ion. Thus, in the chapter dealing with the discovery, development, and chemistry of glyphosate published by Grossbard et al., supra, Franz states that "[a]fter penetration into leaf tissue, therefore, glyphosate exists in the apoplast primarily in its monoanionic form and is translocated via the phloem as the dianion. The similar herbicidal effectiveness of glyphosate acid and many of its soluble salts . . . indicates that the counter-ion may influence formulation solubility but not overall biological activity." (Grossbard et al., supra, p. 9)